The LOCA event starts with a decrease and then the loss of coolant flow due to a break in the coolant pipe; at the same time the reactor is depres­surized, scrammed and shut down (Strasser et al, 2010b). The fuel starts heating up due to its decay heat until the emergency core cooling systems (ECCSs) are activated and fuel cooling commences. Hypothetical LOCA events are analyzed for each reactor to ensure that the safety criteria, as defined by the regulators for the reactor system and the fuel, are met. The design basis accidents (DBAs) which are analyzed fall into two gen­eral categories. The large break, or large break loss of coolant accident (LBLOCA), assumes a double ended break of a primary coolant cold leg of a PWR or a break in the recirculation pump intake line of a BWR, either of which could cause the loss of all the coolant from the core. The small break, or small break LOCA (SBLOCA), assumes a break in one of the smaller primary circuit lines that will cause less coolant loss than the LBLOCA.

The effect of a LOCA cycle on the fuel is shown schematically in Fig. 5.9, plotting the fuel and cladding temperatures as a function of time in the acci­dent (Strasser et al. , 2010b). The loss of coolant flow and reactor pressure at the initiation of the accident will decrease heat transfer and allow the fuel and cladding to heat up until the reactor scrams. The fuel will then cool

о
400
Coolant blockage
50	100	150

Time (s)

5.9 Typical LOCA in a PWR (Strasser et al., 2010b).

down somewhat, partly due to cooling by the steam-water mixture that is

formed, but the cladding temperature will continue to rise.

During and after the LOCA it must be ensured that (Strasser et al,

2010b):

• The core remains coolable (which means that the maximum allowable coolant blockage is limited)

• No fuel dispersal occurs (which means that cladding rupture is not allowed; it is assumed that the cladding burst is so small that only fission gases are released)

• Less than 10% of the fuel rods in the core fail through burst (but without fuel dispersal) (a requirement in Germany only).